The present invention relates to a method of forming a pattern in an oriented thin film such as a zinc oxide thin film, and to an oriented thin film pattern.
In recent years, a thin film pattern obtained by forming a pattern in an epitaxial or oriented thin film has been used in various devices.
Specifically, a thin film with a crystal structure belonging to the hexagonal system, for example, a zinc oxide thin film containing zinc oxide as the main component, is widely used as a piezo-electric thin film, an optical wave guide, a transparent conductive film, or a gas sensitive film. In a case where the zinc oxide thin film is used for the above purposes, it is necessary to form a fine pattern in the zinc oxide thin film. Devices which employ a thin film pattern made of zinc oxide, are discussed on pages 639 to 642 of the Proc. of IEDM, 1983.
In general, a fine pattern is formed in a thin film in the following manner. That is, a resist pattern is first formed on the thin film through photolithographic techniques, and then dry etching or wet etching is carried out for the thin film using the resist pattern as a mask. Further, in a case where a fine pattern is formed in a thin film with a crystal structure belonging to the hexagonal system such as a zinc oxide thin film, the thin film has hitherto been etched by one of isotropic etchants such as hydrochloric acid and nitric acid.
When the above thin film is etched by the isotropic etchant, however, that portion of the thin film under the mask, is over-etched, that is, side etching is generated. Thus, there arises a problem that the patterning accuracy is lowered.
Further, in a case where a line pattern is formed in an oriented thin film such as a zinc oxide thin film by the above method, to be used as the thin film elements of a thin film array ultrasonic transducer, there arise the following problems. That is, the dimensional accuracy of each thin film element is lowered, and variations in area of thin film element are increased. Thus, the sensitivity of respective elements vary widely, and moreover an undesirable response such as a side lobe is generated in an ultrasonic beam.